An access control technique employing a cache has been widely used as a technique to improve a readout performance from a memory device. In this technique, part of data in the memory device is temporarily held in the cache. If particular data is held in the cache at the time when a request for reading out the particular data is received, the data is read out of the cache instead of the memory device.
Meanwhile, there are also known techniques to increase failure resistance by duplicating data stored in a cache. For example, there is proposed a technique to store write data in any one of two cache units and in any one of two non-volatile memory units. In the meantime, there is also proposed a technique to complete a write command at a point of time when write data from a host computer to a main drive is written in both a disk cache and a backup memory. Such related art has been disclosed in Japanese Laid-open Patent Publications Nos. 5-158797 and 4-84215.